Everyone gets confused by this it seems. You can mutate fields in the enclosing class. You can read locals. If you really want to, you can box a local, and then mutate it inside the box. You can't mutate locals directly, because that would make lambdas significantly slower for very little benefit.

(You would not be able to do stack allocation of locals in frames that contain lambdas (unless you could prove that the lambda does not escape).)